The optical sheet exercises predetermined optical functions, such as a function to refract light rays such that the light rays have a peak direction parallel to a normal to the optical sheet, and a function to change a luminance distribution. The optical sheet is incorporated into a screen included in a projection television system or a microfilm reader.
A projection television system is, for example, an image display apparatus including a rear-projection screen that transmits an image projected thereon from behind and displays an enlarged image for viewing from the front side of the rear-projection screen. A general rear-projection screen employed in such a projection television system is used, in most cases, in combination with optical sheets, such as a diffusing sheet that uniformly diffuses light rays transmitted by the rear-projection screen and a lenticular lens sheet that refracts the transmitted light rays in a direction parallel to a normal to the rear-projection screen.
A lenticular lens sheet, namely, an optical sheet, for example, is provided on one of the surfaces thereof with lenses having a semicircular or semielliptic cross section. The optical sheet represented by such a lenticular lens sheet in the shape of a film is manufactured by manufacturing methods such as: (1) a T-die extrusion forming method that rapidly cools, compresses and extrudes through a T die a heated and softened transparent or semitransparent molten resin base and forms lenses in a surface of the molten resin base by imparting a lens shape formed in the circumference of a forming roll to the molten resin base; or (2) a press forming method that presses a transparent or semitransparent molten resin base with a forming die to impart a lens shape formed in a surface of the forming die to the transparent or semitransparent molten resin base to form lenses in the surface of the molten resin base. A photopolymer method such as disclosed in JP 3-127041 A is another method of manufacturing a film-shaped optical sheet. The photopolymer method spreads a radiation-curable resin on a base film, forms lenses by imparting a lens shape formed in a circumference of a forming roller to the radiation-curable resin, and cures the radiation-curable resin by irradiation with ultraviolet rays or the like.
Since the foregoing film-shaped optical sheet is not rigid, is unable to maintain its shape and is liable to deform and cannot be individually installed at a predetermined position in a projection television system or the like. Therefore, the film-shaped optical sheet needs to be installed in combination with a plate-shaped auxiliary structure, such as a rigid sheet. Large display apparatuses are demanded in recent years and large optical sheets for projection television systems and the like are used. Consequently, the importance of rigid sheet for use in combination with optical sheets has progressively grown. Although the optical sheet itself is inexpensive, the costs of the total sheet increases because the optical sheet needs an auxiliary structure, such as a rigid sheet, is difficult to install and needs additional parts.
A previously proposed rigid optical sheet, which is resistant to deformation, is formed in a big thickness to increase rigidity. Such a rigid sheet does not need any auxiliary structure and can be individually installed at a predetermined position in a projection television system or the like.
The rigid optical sheet, however, cannot be efficiently mass-produced and is costly. Although the lens sheet having a rounded surface shape like that of a lenticular lens sheet, can be formed in continuously manufacturing rigid optical sheets by extrusion, a Fresnel lens sheet and a prism structure having a sharp surface shape, and precision lens structures including a fine matte structure, a hairline structure and a diffraction grating structure cannot be formed. When an optical sheet is formed by an extrusion forming method and the molten resin base is cooled by a pair of forming rollers, the molten resin base deforms due to the contraction of the cooled resin, i.e. the returning of formation of the resin, if the molten resin base has a big thickness. Consequently, the accuracy of shaping decreases.
Therefore, the rigid optical sheet, in general, is manufactured by the following sheet-fed manufacturing method. The sheet-fed manufacturing method forms a resin base by shaping a plate with a forming die having a forming surface of a shape complementary to the sharp surface shape of a lens structure or a minute surface shape of a fine matte structure, a hairline structure or a diffraction grating structure by press forming, injection molding, UV molding (ultraviolet molding) or casting, and then removes the shaped resin base from the forming die. The sheet-fed manufacturing method manufactures resin bases at low productivity and high costs.